The invention relates to bubble jet printing system and, more particularly to a printhead which is constructed so as to effectively control heat generated during the printing operation.
Bubble jet printing is a drop-on-demand type of ink jet printing which uses thermal energy to produce a vapor bubble in an ink-filled channel that expels a droplet. A thermal energy generator, usually a resistor, is located in the channels near the nozzle a predetermined distance therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink is ejected from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separting of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
A problem with prior art printhead operation is the increase in temperature experienced by a printhead during an operational mode. With continued operation, the printhead begins to heat up, and the diameter of the ink droplet begins to increase resulting in excessive drop overlap on the recording media thereby degrading image quality. As the printhead experiences a further heat buildup, the ink temperature may rise to a point where air ingestion at the nozzle halts drop formation completely.
Various techiques are known in the prior art to control this heat buildup and maintain the printhead within a reasonable operating temperature range. U.S. Pat. No. 4,496,824 to Kawai et al discloses a thermal printer which includes circuitry to measure printhead temperature, compare the temperature to values representing a desired temperature range and reduce the printhead temperature by activation of a cooling mechanism.
In. U.S. Pat. No. 4,636,812, to Bakewell, a heater and heat sensor operate in combination with a temperature regulator to maintain the printhead at a predetermined temperature.
U.S. Pat. No. 4,571,598 discloses a thermal printhead in which a heat sink and ceramic substrate are connected to heating elements formed on the substrate surface.
These prior art heat management techniques may not be suitable for some printing systems depending on factors such as print head geometry, print speed etc. The present invention is directed towards an ink jet printer which has been modified so as to be optimized for heat management by incorporating heat dissapating elements into the printhead structure. The various heat flow paths taken by heat generated during printhead operation suggested certain modifications which were made to cartridge- type printhead assemblies to enhance heat flow away from the printhead. These modifications include placing heat sink members at optimum locations on both printhead and electrode boards bonded to the printhead. Another modification was the provision of plating holes placed through the electrode board and terminating against the printhead. In a still further embodiment, the heat-dissipating properties of the printhead cartridge were increased by metallizing the body of the cartridge and by adding a heat sink member to the cartridge surface. More particularly, the invention is directed, in a first embodiment to an ink jet printing device for a drop-on-demand thermal ink jet printer, the printing device which includes an ink supply cartridge having a printhead mounted within the cartridge, said printhead being of the type having a plurality of parallel channels, each channel being supplied with ink and having one open end which serves as an ink droplet ejecting nozzle, a heating element being positioned in each channel a predetermined distance from the nozzle, ink droplets being ejected from the nozzles by the selective application of current pulses to the heating elements in response to digitized data signals received by the printing device, the heating elements transferring thermal energy to the ink in contact therewith causing the formation and collapse of temporary vapor bubbles that expel the ink droplets, said print device further comprising an electrode board bonded to said printhead, said electrode board including at least one heat dissipating mechanism.